This invention concerns an improvement to heat pumps and refrigerators of the absorption type. As known, such systems basically operate by supplying heat to an external fluid A while receiving heat from an external fluid B and exchanging heat with an external fluid C. The two cases are to be distinguished. In the first case (called case 1), the thermal level of fluid B is higher than the thermal level of fluid A and the system receives heat from the fluid C which is at a lower thermal level than the fluids A and B. If fluid C is at a thermal level higher than room temperature, the device operated according to such a cycle is usually referred to as an absorption heat pump. In that case, there is transferred to fluid A a heat amount Q.sub.2 higher than the heat amount Q.sub.1 which is transferred from fluid B, and a heat amount Q.sub.2 -Q.sub.1 is received from fluid C. The fluid C may be at a thermal level lower than room temperature; in that case, the device operates as a refrigerator. In the second case (called case 2), the thermal level of fluid B is lower than the thermal level of fluid A and the system supplies heat to fluid C which is at a lower thermal level than fluids A and B: this is a heat converter such as is disclosed in French Pat. No. 2,321,098 (EN No. 7,525,598). In that case, fluid A receives a heat amount Q.sub.2 lower than the heat amount Q.sub.1 which is released by fluid B, and a heat amount Q.sub.2 -Q.sub.1 is supplied to fluid C.
In the description of the present invention, any device receiving heat only above room temperature and complying with the above general definition, either in case 1 or in case 2, is designated as an absorption heat pump.
In the above general description of the prior art, and in the following disclosure, temperature or thermal level is intended to designate more or less wide temperature ranges which may be also a substantially constant temperature in the case of a change of state of the external fluid with which the heat exchange is effected. A first "thermal level" is considered as higher than a second "thermal level" if the corresponding temperature range is at least partly above the temperature range of the second "thermal level".
In the two cases described above, the cycle comprises at least one absorption step in which a gas phase of a working fluid (the solute) is contacted with a liquid phase (the solvent), and a desorption step which yield a liquid phase of lower solute content and a gas phase of high solute content. Therefore, the solute is a material which may appear either as a gas or in the dissolved state.
The absorption step is usually effected in one contact step; the liquid solvent phase (L) and the gas solute phase (V) are fed to an enclosure and a solution (S) is obtained, while the absorption heat is discharged by indirect contact exchange in the absorber, as disclosed in FIG. 1A.
It is also known to effect the absorption step by counter-current contact of the liquid phase with the vapor phase. This system is disclosed, for example, in the French Pat. No. 2,321,098. In that case, it is possible to operate according to the arrangement shown in FIG. 1B. The counter-current contact is effected in adiabatic conditions, for example in a column, and the discharge of heat to the environment is effected at least partly by condensation, outside the column, of the vapor obtained at the top. In the contact column, the absorption heat results in the vaporization of a fraction of the solvent phase which is less volatile, and there is collected, at the top of the column, a vapor fraction of higher solvent vapor content, which fraction is condensed in exchanger (W) while supplying heat to fluid A.
Adiabatic conditions are conditions in which, as a result of the limitation or even absence of heat exchange with the environment, any released heat is recovered, at least in part, in the fluids discharged from the zone subjected to these conditions.
If the solvent phase fed to the column is in the pure state, it is possible to collect, at the top of the column, a vapor fraction of low solute content and high solvent content, and to thus obtain, at the top of the column, a temperature higher than at the bottom, also higher than the temperature which is attained in one single contact stage.
This arrangement has also some disadvantages. In fact, the condensation range for the top vapor depends on its content of solute; therefore, to avoid that the final condensation temperature be far lower than the initial condensation temperature, it is necessary to allow only a low proportion of solute in the top vapor and therefore the solvent phase must be totally purified in the desorption step.
This problem is made apparent when considering ammonia absorption in aqueous phase in the case where the vapor is discharged in a practically balanced proportion to the solvent phase (L) supplied to the column, which occurs when the vapor feed rate is low as compared to the liquid feed rate. The following results are obtained when varying the composition of the solvent phase (L) at a pressure of 10.2 atm.
TABLE 1 ______________________________________ NH.sub.3 NH.sub.3 Initial Final molar molar conden- conden- fraction fraction Heat of sation sation in the in the conden- temper- temper- Condensa- solvent vapor sation ature ature tion Range phase phase cal/mole .degree.C. .degree.C. .degree.C. ______________________________________ 0.01 0.065 8735 178 162 16 0.04 0.242 8933 169 118 51 0.10 0.512 8740 153 64 89 ______________________________________
It is apparent that the condensation range widens very quickly. To avoid this it is necessary to thoroughly purify the solvent phase in the desorption step and to limit the feed rate of the vapor fraction fed to the column. These requirements limit the yield.